Fully override default slice fields of config (open-telemetry#4001)

mapstructure library doesn't override full slice during unmarshalling.
Origin issue: mitchellh/mapstructure#74 (comment)
To address this we empty every slice before unmarshalling unless user provided slice is nil.

.chloggen/config-slice-fields.yaml

@@ -0,0 +1,5 @@
+change_type: "bug_fix"
+component: "config"
+note: "Fully override default slice fields of config"
+issues: [4001]
+subtext:

confmap/confmap.go

@@ -168,6 +168,7 @@ func decodeConfig(m *Conf, result interface{}, errorUnused bool) error {
 			mapstructure.StringToTimeDurationHookFunc(),
 			mapstructure.TextUnmarshallerHookFunc(),
 			unmarshalerHookFunc(result),
+			zeroesSliceHookFunc(),
 		),
 	}
 	decoder, err := mapstructure.NewDecoder(dc)
@@ -320,6 +321,23 @@ func marshalerHookFunc(orig interface{}) mapstructure.DecodeHookFuncValue {
 	}
 }
 
+// mapstructure library doesn't override full slice during unmarshalling.
+// Origin issue: https://github.com/mitchellh/mapstructure/issues/74#issuecomment-279886492
+// To address this we empty every slice before unmarshalling unless user provided slice is nil.
+func zeroesSliceHookFunc() mapstructure.DecodeHookFuncValue {
+	return func(from reflect.Value, to reflect.Value) (interface{}, error) {
+		if from.Kind() == reflect.Slice && from.IsNil() {
+			return from.Interface(), nil
+		}
+
+		if to.CanSet() && to.Kind() == reflect.Slice {
+			to.Set(reflect.MakeSlice(reflect.SliceOf(to.Type().Elem()), 0, 0))
+		}
+
+		return from.Interface(), nil
+	}
+}
+
 // Unmarshaler interface may be implemented by types to customize their behavior when being unmarshaled from a Conf.
 type Unmarshaler interface {
 	// Unmarshal a Conf into the struct in a custom way.

confmap/confmap_test.go

@@ -400,3 +400,134 @@ func TestUnmarshalerErr(t *testing.T) {
 	assert.EqualError(t, cfgMap.Unmarshal(tc), expectErr)
 	assert.Empty(t, tc.Err.Foo)
 }
+
+func TestUnmarshalerSlices(t *testing.T) {
+	type innerStructWithSlices struct {
+		Ints []int `mapstructure:"ints"`
+	}
+	type structWithSlices struct {
+		Strings []string              `mapstructure:"strings"`
+		Nested  innerStructWithSlices `mapstructure:"nested"`
+	}
+
+	tests := []struct {
+		name     string
+		cfg      map[string]any
+		provided any
+		expected any
+	}{
+		{
+			name: "overridden by slice",
+			cfg: map[string]any{
+				"strings": []string{"111"},
+			},
+			provided: &structWithSlices{
+				Strings: []string{"xxx", "yyyy", "zzzz"},
+			},
+			expected: &structWithSlices{
+				Strings: []string{"111"},
+			},
+		},
+		{
+			name: "overridden by a bigger slice",
+			cfg: map[string]any{
+				"strings": []string{"111", "222", "333"},
+			},
+			provided: &structWithSlices{
+				Strings: []string{"xxx", "yyyy"},
+			},
+			expected: &structWithSlices{
+				Strings: []string{"111", "222", "333"},
+			},
+		},
+		{
+			name: "overridden by an empty slice",
+			cfg: map[string]any{
+				"strings": []string{},
+			},
+			provided: &structWithSlices{
+				Strings: []string{"xxx", "yyyy"},
+			},
+			expected: &structWithSlices{
+				Strings: []string{},
+			},
+		},
+		{
+			name: "not overridden by nil slice",
+			cfg: map[string]any{
+				"strings": []string(nil),
+			},
+			provided: &structWithSlices{
+				Strings: []string{"xxx", "yyyy"},
+			},
+			expected: &structWithSlices{
+				Strings: []string{"xxx", "yyyy"},
+			},
+		},
+		{
+			name: "not overridden by nil",
+			cfg: map[string]any{
+				"strings": nil,
+			},
+			provided: &structWithSlices{
+				Strings: []string{"xxx", "yyyy"},
+			},
+			expected: &structWithSlices{
+				Strings: []string{"xxx", "yyyy"},
+			},
+		},
+		{
+			name: "not overridden by missing value",
+			cfg:  map[string]any{},
+			provided: &structWithSlices{
+				Strings: []string{"xxx", "yyyy"},
+			},
+			expected: &structWithSlices{
+				Strings: []string{"xxx", "yyyy"},
+			},
+		},
+		{
+			name: "overridden by nested slice",
+			cfg: map[string]any{
+				"nested": map[string]any{
+					"ints": []int{777},
+				},
+			},
+			provided: &structWithSlices{
+				Nested: innerStructWithSlices{
+					Ints: []int{1, 2, 3},
+				},
+			},
+			expected: &structWithSlices{
+				Nested: innerStructWithSlices{
+					Ints: []int{777},
+				},
+			},
+		},
+		{
+			name: "overridden by weakly typed input",
+			cfg: map[string]any{
+				"strings": "111",
+			},
+			provided: &structWithSlices{
+				Strings: []string{"xxx", "yyyy", "zzzz"},
+			},
+			expected: &structWithSlices{
+				Strings: []string{"111"},
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		tt := tt
+
+		t.Run(tt.name, func(t *testing.T) {
+			cfg := NewFromStringMap(tt.cfg)
+
+			err := cfg.Unmarshal(tt.provided)
+			if assert.NoError(t, err) {
+				assert.Equal(t, tt.expected, tt.provided)
+			}
+		})
+	}
+}